Correction for Inertia. 675 It is convenient to make one more scale, to show pressure per square inch of piston. The piston area being 491 square ins., divide the total pressure reading by 491 to obtain reading per sq. in.; stepped off at s/. The indicator card for the top of the piston is set out by the unit pressure scale at s/, and appears as EQXHB, the bottom of diagram touching the base AB. Similarly FPGA is the card from the bottom of the piston. Now, while QXHB is being drawn by the indicator on top side of piston, A F R would be formed by that connected with the bottom side, and the -effective pressure will be the difference of these curve ordinates. Deduct those at F from those at H, and the result is the curve WR. So also VN is the curve of effective pressure on the bottom side of the piston. Now the actual total pressure to be carried forward to the crank pin will be, during the first half of the stroke, less than that on the indicator diagram by the amount required to set in motion the reciprocating masses, viz., their inertia; and during the second half of stroke the indicated pressure will be increased by the backward pull needed to absorb inertia. Briefly, then, the c top' card loses by the area ANS, and gains by SBP, the resulting pressure area being NXWP; and similarly the resulting area for the 'bottom' card will be P/VN. Setting up the resulting ordinates on the straight base AB, we have the curve AŁ*/B for the top and B <?/A for the bottom of piston, the total pressures being written on each ordinate; and in order to equalise the areas the cut-off in top diagram has been placed at '3 and in bottom at '6 of stroke, the dead weights having to be supported in the latter case. We must next distinguish between reciprocating and rotating parts, for only the former cause inertia force. The piston, piston rod, crosshead, and smaller end of connecting rod are recipro- cating weights, but the larger end of connecting rod is a rotating weight As regards the connecting rod itself, about two-thirds may be called reciprocating and the remaining third reckoned as a rotative weight. The reciprocating weights directly affect the indicator diagram, and the latter must be altered, by increased compression or later cut-off, until a fairly even pressure is